Faulty sensor causes stalling

Question: I have a 1988 Oldsmobile Cutlass equipped with a fuel injected 2.8-L V6 engine and automatic transmission. Over the last few months the engine would sometimes die when I came to a stop.

I have had it to a couple repair shops and have had the throttle plates cleaned and the idle air control valve replaced. Engine idle speed is correct and there are no codes stored in the engine computer. The stalling problem is getting worse. What should I look at to fix my car?

Answer: When everything checks out fine on this car and it still stalls, the mass air flow sensor is a prime suspect. The mass air flow sensor tells the fuel injection computer how much air is entering the engine and then the computer can inject the correct amount of fuel. If a false signal comes from the sensor, then the computer doesn't know which sensor to take readings from and the engine stalls.

The computer is supposed to recognize a fault with the mass air flow sensor and set a trouble code, but sometimes the fault occurs for such a short time that no code is set. Tapping on the air flow sensor (it is in the air duct between the air cleaner and the throttle plates) is one test of the sensor. If the engine starts to run rough or stalls, then the sensor is faulty. The actual sensor signal can be measured but a lab scope is necessary to pick up quick signal variations that indicate a bad sensor.

Instead of replacing a faulty mass air flow sensor, this year and model of car is one of the few that can be converted to a "speed density system". The conversion involves installing a new program chip in the computer and removing the power relay for the mass air flow sensor. Now the computer uses information from its other sensors to calculate the air flow. The mass air flow sensor is disabled and only remains in place to complete the ducting from the air cleaner to the engine.